1. Field of the Invention
This invention generally relates to the field of converting molten sulfur (or sulphur) into solid pellets (or prills).
2. Description of the Related Art
Sulfur is used in numerous products, including fertilizers, gunpowder, insecticides, fungicides, paper, and textiles. It may be extracted directly from the earth, or it may be removed from other natural substances, such as coal, natural gas, or crude oil. Liquid or molten sulfur produced as a by-product from petro-chemical refineries often contains particulate impurities known as “Carsul,” which is a carbon sulfur polymer. Sulfur is usually produced, transported, and utilized within the United States in molten liquid form. It is inconvenient and expensive to store and transport sulfur in a molten form. In addition, before sulfur can be exported, generally, it has to be converted to solid form.
The prior art contains proposals for converting liquid or molten sulfur into pellets (known sometimes as “prills”). For example, “Wet” processes propose forming and solidifying droplets of sulfur in a liquid coolant. U.S. Pat. No. 4,149,837 proposes converting molten sulfur into solid pellets by passing droplets through a liquid medium at temperatures in excess of 150° F., and preferably with the liquid medium approaching the melting point of sulfur.
U.S. Pat. No. 3,649,217 proposes separating sulfur from crushed ore by passing the crushed ore containing sulfur through a hot water section in the range of 290°-320° F. that melts the sulfur, and then through a cold water section in the range of ambient to 150° F. According to the '837 patent, the quenched pellets are usually hollow, pocked with pin holes, resemble somewhat pieces of popcorn, crumble when extensively handled, and tend to retain moisture.
U.S. Pat. No. 4,133,669 proposes forming prills from a molten sulfur-bentonite mixture by passing the mixture through a liquid cooling medium comprising liquid fertilizer. The '669 patent proposes that if the liquid cooling medium was water, or contained an excessive amount of water, the mixture would not become pelletized, but would degrade and turn into “mush” as the mixture entered the water.
For maximum commercial value, sulfur prills should be generally spherical in shape, uniform in size and density, and have low moisture content. Very small or fine pieces of sulfur, commonly known as “fines,” are undesirable and create an enormous maintenance problem and potential fire and safety hazards and health problems for manufacturing personnel. High moisture content is undesirable because, among other things, the customer typically pays by weight, and less sulfur is received. Further, the increased weight increases the cost of shipping, and water and sulfur may create dangerous sulfuric acid. Sulfur prills typically should have a moisture content of 1.8% to 2.2% to command maximum commercial value and be acceptable for export. Moreover, the prills typically should meet the size and uniformity criteria set forth below in Table 1.
TABLE 1Shape, size, and size distribution:Generally smooth and free of angularities90 percent must remain on #4 Tyler Mesh screen (4.75mm) to #16 mesh screen (1.18 mm)No material should exceed 6 mm in diameterNo more than 10 percent retained on the #4 mesh screenNo more than 10 percent should pass the #16 mesh screenNo more than 2 percent passing a 50 mesh screen
A need exists for a method and system to convert liquid or molten sulfur to solid prills that produces prills meeting the size, uniformity, and moisture content criteria generally described above for maximum commercial value. The method and system would use an economical novel wet process of quenching the sulfur through a liquid medium, such as water.